Interpretive Summary: A collaborative study was conducted involving scientists from the Livestock Issues Research Unit, Lubbock, TX, the USDA-ARS SubTropical Agricultural Research Station in Brooksville, FL, the University of Missouri, the University of Florida’s Range Cattle Research and Education Center at Ona, and Texas Tech University to evaluate two Bos taurus breeds with known differences in heat tolerance under controlled conditions to evaluate heat tolerance. Angus and Romosinuano steers were tested in thermoneutral and mild heat stress environments to determine differences between heat sensitive and heat tolerant Bos taurus breeds. Our results indicated that in both environments, Angus cattle exhibited a greater heat loss than the heat tolerant breed. Surprisingly, the Angus breed also exhibited the greater heat load. Blood analyses showed that prolactin, creatinine, and cholesterol were more responsive to the increased load and might possibly be used to identify sensitivity to heat stress in cattle. The present study has identified additional physiological and endocrine markers that may aide in the identification of Bos taurus sensitivity to heat. The reasons for these changes during heat stress remain to be determined and are continuing to be explored by this research team. The results of this research will be of particular interest to beef cattle feedlot managers, veterinarians managing the health of feedlot cattle, and scientists, whether from industry, academia, or industry, working in the area of beef cattle production, health, and well-being.

Technical Abstract:
Two Bos taurus breeds with known differences in heat tolerance were tested under controlled conditions to evaluate heat tolerance. Romosinuano (RO) is a tropically adapted breed. Nine Angus (304 ± 7 Kg BW; AG) and nine RO (285 ± 7.5 Kg BW) steers from USDA-ARS, Brooksville, Florida, were transported to the Brody Environmental Center at the University of Missouri. Steers were housed for 14 days at thermoneutrality (21C; TN) before 14 days of cyclic heat stress (HS; 26C night; 36C day). Rectal temperature and respiration rate were measured six times daily. Sweat rates at shaved sites were recorded on specific days. Blood samples were taken once a week. The RO maintained a lower respiration rate (20 bpm), sweat rate (6 g/m2/h), and rectal temperature (0.5C) than AG throughout TN. Both breeds increased sweat rate, respiration rate, and rectal temperature during HS, with AG retaining the higher levels. There were breed differences for serum prolactin, leptin, creatinine, and cholesterol, with AG being higher than RO. Serum leptin increased for both breeds with HS. Although there were no breed differences at TN, AG steers exhibited HS-induced increases in prolactin, creatinine, and cholesterol. However, these measures for RO were unaffected by HS. The present study has identified additional physiological and endocrine markers that may aide in the identification of Bos taurus sensitivity to heat. The reasons for these changes during heat stress remain to be determined.